Species differences in histamine receptors in the vas deferens

Histamine, specific H₁-and H₂-receptor agonists in conjunction with specific H₁-and H₂-receptor antagonists and other types of classical antagonists were used to characterize histamine receptors in the vasa deferentia of mice, rats and guinea pigs. The H₁-receptor mediates contraction while the H₂-receptor produces inhibition. There were marked qualitative and quantitative differences in the distribution of the two types of histamine receptors in the vas deferens of different species. Results indicate that mouse and rat vas deferens contain an inhibitory H₂-receptor, but virtually no excitatory H₁-receptor. In contrast, guinea pig vas deferens contained an excitatory H₁-receptor but was essentially devoid of an inhibitory H₂-receptor. The rank order of relative potencies of various agonists as well as the calculated p A₂ values of cimetidine in the mouse and rat vas deferens suggest that the two species probably have the same H₂-receptor. High concentrations of histamine and 2-methyl histamine have a stimulant action in the mouse and rat vas deferens which was secondary to release of endogenous noradrenaline rather than to the stimulation of an excitatory H₁-receptor.